Updater: allow re-check after checking

When opening the activity update control panel, closing it once the list
of available updates has been presented, and then trying to open it again,
we would be in STATE_CHECKED and try to check for updates again
(i.e. refresh).

This was causing an UpdaterStateException even though we can and should
handle this simple refresh case.
Microformat update: fix size lookup handling

For a Downloader.get_size request, the size of the remote file is
passed in the result parameter. Not sure why we were trying to call
the result as if it were callable.

Fixes a failure to update activities using the microformat updater.
View Source: don't crash if toolkit path wasn't found - SL #4605

Log an error instead.

Toolbar class was considering a value of None for the path, but the
ViewSource class wasn't.
View Source: display toolkit source again - SL #4605

In case of a GTK activity, it will display the source of
sugar-toolkit-gtk3 .  We can enhace this later, if we could check if
the activity is GTK2 or GTK3.  There is a FIXME note in the code for
this.

In case of a web activity, it will display sugar_web inside the
activity bundle.  The initial selected file will be env.js, which is
analog to the initial file env.py in sugar-toolkit-gtk3 .
View Source: initiate with index.html for web activities

A better option would be js/activity.js, but that implies bigger
changes in FileViewer class, and we are in freeze.